Virtual memory is a term applied to memory systems that allow programs to address more memory than is physically available. Disk storage provides the increased memory by storing data that is not currently being accessed. When data in the disk storage is referenced, the operating system moves data resident in memory to the disk storage, and moves the referenced data from the disk storage into memory. This moving of data between memory and disk storage is called demand paging.
One or more translation tables are typically used for translating the virtual address to a corresponding physical address. For example, the virtual address may be subdivided into a segment number that indexes a segment table, a page number that indexes a page table selected by the indexed entry in the segment table, and a byte offset. In this case, the indexed entry in the page table provides a physical page number, and the physical address is the concatenation of the physical page number and the byte offset. To reduce the time for translating virtual addresses to physical addresses, the most recently used virtual-to-physical address translations can be cached in a high-speed associative memory called a translation buffer. See Henry M. Levy and Richard H. Eckhouse, Jr., Computer Programming and Architecture, The VAX-11, Digital Equipment Corporation, 1980. pp. 250–253, 358–360.
Recently memory has become so inexpensive that it is often desirable for a processor to access more memory than can be addressed in a given virtual address space. For example, the virtual memory address in many microprocessors is limited to 32 bits, so that the virtual address space has a size of four gigabytes. One technique for permitting a 32-bit virtual address to access more than four gigabytes of physical memory is the physical address extension (PAE) feature introduced in the Intel Pentium Pro processor and included in other Intel P6 processors. The PAE feature provides generic access to a 36-bit physical address space by expanding page-directory and page-table entries to an 8-byte (64 bit) format, and adding a page-directory-pointer table. This allows the extension of the base addresses of the page table and page frames from 20 bits to 24 bits. This increase of four bits extends the physical address from 32 bits to 36 bits.